Adenosine Cyclic 3′:5′-Monophosphate-dependent Protein Kinase from Rabbit Erythrocytes

Adenosine cyclic 3':5'-monophosphate (cyclic AMP)-dependent protein kinase from rabbit erythrocytes has been resolved into three active fractions possessing both phosphotransferase and cyclic AMP-binding activities. These enzymes are arbitrarily designated as kinases I, IIa, and IIb. The kinetic properties and substrate specificities of these kinases are remarkably similar. All three variants catalyze the phosphorylation of calf thymus histones and are stimulated by low concentrations of cyclic AMP. The molecular weights of these enzymes have been estimated by gel filtration to be 170,000, 120,000, and 240,000, corresponding to kinases I, IIa, and IIb, respectively. The three kinases differ in temperature (53°) stability. Kinases IIa and IIb are inactivated more slowly than kinase I. However, the rate of inactivation of kinase I is greatly diminished in the presence of both ATP and Mg2+. A similar protection by these compounds is not observed with kinases IIa and IIb. Each of these kinases appears to be constructed of two dissimilar functional subunits: a catalytic subunit and a regulatory subunit. Cyclic AMP causes the two dissimilar functional subunits to dissociate by binding to the regulatory moiety. A similar activation may be accomplished by protamine. In addition, protamine enhances the binding of cyclic AMP to the kinases as determined by the Millipore filtration assay. The regulatory moiety of kinase I cross-reacts with the catalytic moiety of both kinases IIa and IIb.